1. Field of the Invention
The present invention relates to a nanoparticle and a method of manufacturing the same. In particular, it relates to a nanoparticle that can maintain high-emission properties and that has an excellent durability, and a method of manufacturing the same. The invention also relates to a fluorescence reagent and an optical device comprising the nanoparticle.
2. Background Art
Nanoparticles such as semiconductor nanoparticles are characterized in that they can emit narrow and strong fluorescence of the full width at half maximum (FWHM). These are materials that have been gaining much attention, as they are capable of producing various fluorescent colors and are thought to provide an extremely wide range of applications in the future.
Because the semiconductor nanoparticles with particle sizes of not more than 10 nm are located in the transition region between bulk semiconductor crystals and molecules, they exhibit physicochemical properties that are different from those of either the bulk semiconductor crystals or molecules. In such a region, the degeneracy of energy bands that is observed in bulk semiconductors is removed and the orbits become discrete, and the quantum size effect appears in which the energy width in the forbidden band changes depending on the particle size. Due to the appearance of the quantum size effect, the width of energy in the forbidden band decreases or increases in response to an increase or decrease of the particle size. The change in the energy width in the forbidden band influences the fluorescent properties of the particle. A particle with a small particle size and a large energy band in the forbidden band has fluorescent wavelengths more towards the shorter wavelengths, while a particle with a large particle size and a smaller energy band in the forbidden band has fluorescent wavelengths more towards the longer wavelengths. Namely, the semiconductor nanoparticle is a material capable of producing any desired fluorescent colors by controlling the particle size, hence the attention it is gaining.
In order to utilize the semiconductor nanoparticle as a fluorescent material, the particle size must be controlled. In addition, if the particle size could be monodispersed, it could be expected that semiconductor nanoparticles would be prepared that have preferable fluorescent properties exhibiting a spectrum profile with narrow FWHM.
The semiconductor nanoparticle manufacturing process can be roughly divided into the preparation of particles and the monodispersion of particle sizes. Particles can be easily prepared by dissolving equimolecular amounts of precursors of Cd and X (X is S, Se, Te). This is the same for the manufacture using CdSe, ZnS, ZnSe, HgS, HgSe, PbS, PbSe, and so on. The thus prepared semiconductor nanoparticles exhibit a wide particle distribution to which a technique has been further attempted to monodisperse the particle distribution. For example, particle separation is conducted with high accuracy using a chemical technique in order to separate and extract only nanoparticles of a particular particle size. Examples of this method that have so far been reported include the electrophoretic separation method taking advantage of the variation of surface charge of the nanoparticle depending on particle size, the exclusion chromatography taking advantage of the difference in retention time depending on particle size, and the size-selective precipitation method taking advantage of the difference in dispersibility into an organic solvent depending on particle size.
In the aforementioned method, semiconductor nanoparticles prepared by mixing precursors are classified according to particle size. On the other hand, a method has also been reported that attempts to carry out the preparation of particles and the monodispersion of particle sizes at the same time. A typical example is a reversed micelle method. In this method, amphipathic molecules, such as diisooctyl sodium sulfosuccinate, and water are mixed in an organic solvent, such as heptane, thereby forming a reverse micelle in the organic solvent, such that precursors are reacted with each other using only the aqueous phase in the reverse micelle. The size of the reverse micelle is determined by the quantitative ratio of the amphipathic molecules to the water, so that the size can be relatively uniformly controlled. The size of the nanoparticle prepared is dependent on the size of the reverse micelle, so that it is possible to prepare semiconductor nanoparticles with relatively uniform particle sizes. Other methods for simultaneously preparing particles and monodispersing particle sizes are disclosed in J. Phys. Chem. B. 101: 9463 (1997) and JP Patent Publication (Kohyo) No. 2001-523758 A, for example, wherein the Ostwald ripening phenomenon is utilized with the use of trioctylphosphine (TOP) or trioctylphosphineoxide (TOPO). However, the preparation methods disclosed in the above reports are characterized in that a reagent with high toxicity is synthesized at high temperatures and are therefore not necessarily superior from the viewpoint of safety.
An alternative method called size-selective photoetching method takes advantage of photochemical reaction. In this method, the particle sizes in a solution of semiconductor nanoparticles prepared with a wide particle size distribution are monodispersed by utilizing the oxidized-melting of a metal chalcogenide semiconductor upon light irradiation in the presence of dissolved oxygen. For example, when CdS nanoparticles are optically excited in the presence of dissolved oxygen, the excited electrons promote a reduction reaction in which oxygen is reduced, and the holes promote an oxidation reaction in which the CdS nanoparticles themselves are dissolved. This photocatalytic reaction proceeds while the semiconductor nanoparticles are excited. Namely, the dissolving reaction of all of the excited semiconductor nanoparticles ends with the particle size that has a forbidden band width corresponding to the energy of the minimum wavelength of the irradiating light. Specifically, by irradiating the semiconductor nanoparticles having a wide particle size distribution with light with a shorter wavelength than the wavelength of the absorption edge of the semiconductor nanoparticles, semiconductor nanoparticles with large particle sizes can be selectively irradiated and dissolved into smaller, uniform semiconductor nanoparticles. In this method, nanoparticles that are monodispersed at any desired particle size can be relatively safely prepared at room temperature by simply selecting the wavelength of irradiating light. Moreover, by using monochromatic light for irradiation, the monodispersing process can be more accurately performed.
Meanwhile, the inventors' research showed that the quality of the semiconductor nanoparticles that have been monodispersed by the size-selective photoetching method varies greatly. They also showed that the variation is particularly significant in cases where a surface reformulation is provided to the semiconductor nanoparticles. After extensive research and studies, the inventors realized that the pH value during size-selective photoetching is to a large extent involved as a cause of the variation. Namely, it is possible to prepare particles with high reproducibility by controlling the pH value during size-selective photoetching reaction.
Prior to photo-irradiation, the particle size distribution of the semiconductor nanoparticles obtained by the aforementioned preparation methods extends more than 15% of the average particle size in terms of standard deviation. After photo-irradiation, when the irradiation light wavelength is 476.5 nm, the standard deviation (rms) has a very narrow particle size distribution of about 6% of the average particle size.
However, the fluorescent properties of the semiconductor nanoparticles prepared by these methods exhibit a smooth fluorescent spectrum without any peaks. Moreover, the fluorescent spectrum has a peak at a different wavelength than a theoretical value of fluorescence at which the semiconductor nanoparticles are supposed to emit light. Namely, besides the band gap fluorescence emitted from the inside of the semiconductor nanoparticles, the semiconductor nanoparticles emit a totally separate fluorescence for which the energy level that exists in the forbidden band of the energy levels inside the semiconductor nanoparticles is thought to be responsible. These energy levels producing the separate fluorescence are thought to exist mainly in the surface sites of the semiconductor nanoparticles. This is a phenomenon obstructing the properties of the semiconductor nanoparticles with a narrow particle size distribution and has been a problem to be solved, as the change in fluorescent properties caused by controlling the size of semiconductor nanoparticles is supposed to appear in the band gap fluorescence.
In a typical method of solving the aforementioned problem, a semiconductor material as a core is coated with another semiconductor material, an inorganic material, and an organic material that have a wider band gap than that of the core's semiconductor material, thus constructing a layered structure in an attempt to suppress the aforementioned fluorescence.
In typical methods of coating an inorganic material, CdS is coated on a CdSe nanoparticle, as described in J. Phys. Chem. B. 100: 8927 (1996), ZnS is coated on a CdS nanoparticle, as described in J. Phys. Chem. 92: 6320 (1988), and ZnS is coated on a CdSe nanoparticle, as described in J. Am. Chem. Soc. 112: 1327 (1990). With regard to the coating of a CdSe nanoparticle with ZnS as described in J. Phys. Chem. B. 101: 9463 (1997) or JP Patent Publication (Kohyo) No. 2001-523758 A, a semiconductor nanoparticle that has sufficient fluorescent properties has been successfully obtained by adopting a manufacturing method that utilizes the Ostwald ripening phenomenon and that is conducted in a coordination solvent.
The complex-layered semiconductor nanoparticle described above is a material that has a larger band gap than that of the semiconductor nanoparticle. The coating with a substance that does not have a band gap in the forbidden band of the semiconductor nanoparticle is carried out in an attempt to suppress the defective site on the surface of the semiconductor nanoparticle and obtain the inherent fluorescent properties of the semiconductor nanoparticle.
A method of performing a surface treatment in an aqueous solution is disclosed in J. Am. Chem. Soc. 109: 5655 (1987), in which it is reported that the fluorescent properties of the semiconductor nanoparticle in an alkaline aqueous solution has improved. Although various experiments and reports have been made based on this report, none have successfully shed light on the mechanism of such an improvement (J. Phys. Chem. Soc. 100: 13226 (1996) and J. Am. Chem. Soc. 122: 12142 (2000), for example). Moreover, all of the semiconductor nanoparticles in the alkaline aqueous solution have poor reproducibility, such that the conditions for reproduction have not been identified. Furthermore, none of the experiments and reports have successfully isolated a final substance.
As an example of the method of coating with an organic material, there is a synthesizing method that utilizes the Ostwald ripening phenomenon in a coordination solvent. This method employs TOPO (trioctylphosphine) or hexadecylamine (HDA) as the coating material, for example, to obtain semiconductor nanoparticles with high light-emission properties (J. Phys. Chem. B. 101: 9463 (1997)). It should be noted, however, that the final product of the semiconductor nanoparticle is not water-soluble.
The semiconductor nanoparticle obtained by the above-described methods is capable of suppressing a defect site to some extent and has the inherent properties of a semiconductor nanoparticle to some extent. However, in order to prepare such a semiconductor nanoparticle, a highly sophisticated technique is required, and in order to achieve high quality, a variety of equipment is required. Further, they are seriously deficient for the purpose of industrial production in terms of the cost of reagents or the like and the safety during high temperature reaction.
The inventors have conducted research and studies in order to find alternative methods, as well as trying to solve the aforementioned problem. As mentioned above, the surface condition of the semiconductor nanoparticle is thought to be involved in the defective fluorescence of a monolayer semiconductor nanoparticle. Based on this hypothesis, the inventors conducted an analysis of the influence of the surface condition of the semiconductor nanoparticle. As a solution for the relevant defects, the inventors conducted the analysis, focusing on the fact that the emission properties of semiconductor nanoparticles in the aforementioned alkaline aqueous solution are very good. As a result, they eventually succeeded in isolating and purifying semiconductor nanoparticles to which a surface treatment has been conducted in an alkaline solution, and also found a method of rendering the semiconductor nanoparticle water-soluble. Specifically, a semiconductor nanoparticle is given a surface-treating material that provides it with one or more kinds of electron-releasing group, such that the electron-releasing groups are arranged on the surface of the core of the semiconductor nanoparticle, thereby drawing high-emission properties. In this method, the particle can be rendered water-soluble depending on the type of the surface-treating material.
The semiconductor nanoparticle with the high-emission properties prepared by this method, however, is easily influenced by external factors, such as a change in pH.
The semiconductor nanoparticle has such a property that it is more durable than the currently available reagents such as organic pigment, and it hardly fades. Moreover, by changing the particle size, reagents that exhibit a variety of narrow FWHM spectra can be prepared using the same material. Thus, the semiconductor nanoparticle can be applied not only to optical devices but also to biopolymer detection and bioimaging, for example. The semiconductor nanoparticle is thus extremely versatile and is therefore gaining much attention in recent years, and its practical application has been an important issue among the researchers in recent years.